Two-piece oil-cooled piston with thermal expansion control

ABSTRACT

A multi-piece piston assembly including a skirt defining portion and a head defining portion which interfit to provide wrist pin bores in registry for receiving a wrist pin, the skirt defining and head defining portions being composed of a metal having good thermal conductivity such as an aluminum alloy, together with a bridge member which is anchored in the skirt defining portion in the region of the thrust faces to provide thermal expansion control at the top of the skirt portion and to provide additional mechanical strength.

'United States Patent [191 Clary et al.

TWO-PIECE OIL-COOLED PISTON WITH THERMAL EXPANSION CONTROL Inventors:Harry Earl Clary, Chesterland;

Harold Cyril Skopin, Cleveland, both of Ohio Assignee: TRW Inc.,Cleveland, Ohio Filed: Mar. 1, 1972 Appl. No.: 230,741

US. Cl 92/186, 92/216, 92/219,

92/228 Int. Cl. F0lp 3/10, F16j 1/00 Field of Search 92/190, 215, 216,219,

References Cited UNITED STATES PATENTS 8/1925 Nelson 92/225 X 2/1930Noble 92/190 X 451 Apr. 23, 1974 1,929,831 10/1933 Teetor 92/1902,110,346 3/1938 Teetor 92/228 2,180,521 11/1939 Harley 92/228 2,240,9675/1941 Venner et a1. 92/228 Primary Examiner-Irwin C. Cohen Attorney,Agent, or Firm-Hill, Sherman, Meroni, Gross & Simpson [57] ABSTRACT Amulti-piece piston assembly including a skirt defining portion and ahead defining portion which interfit to provide wrist pin bores inregistry for receiving a wrist pin, the skirt defining and head definingportions being composed of a metal having good thermal conductivity suchas an aluminum alloy, together with a bridge member which is anchored inthe skirt defining portion in the region of the thrust faces to providethermal expansion control at the top of the skirt portion and to provideadditional mechanical strength.

3 Claims, 2 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of theInvention This invention is in the field of multi-piece pistons whichare retained in proper working relation with respect to each other bymeans of the wrist pin and the cylinder bore, the skirt portion of thepiston being provided with a bridge member composed of a metal having alower rate of thermal expansion than the metal of the skirt portion.

2. Description of the Prior Art There have been some two-piece pistondesigns utilizing mating skirt portions and head portions which havecertain advantages over one-piece pistons. Each element, for example, isfree to rotate about the wrist pin to the extent that the clearancebetween the cylinder bore and the outside diameters of each of theelements will permit. This feature significantly reduces themisalignment of the ring belt section of the piston head which isencountered with a conventional one-piece pistonwhen side thrust loadsare imposed by the angularity of the connecting rod. In two-piece pistonconstructions, the piston rings are permitted to maintain a moreeffective seal and thereby reduce blow-by and oil consumption.Furthermore, the clearance between the skirt section of the two-piecepiston and the cylinder bore can be less than that which is requiredwith a conventional one-piece piston since heat is not transmitteddirectly from the piston crown to the skirt to cause gross thermalexpansion of the piston skirt.

The difficulty with two-piece piston structures in the past, however,has been a lack of strength which is required to withstand the thermaland mechanical loads encountered in high output engines Furthermore,such pistons normally do not provide any adequate means for cooling thecrown in the ring belt areas. The provision of an improved two-piecepiston which overcomes the difficulties of the prior art structures isthe principal object of the present invention.

SUMMARY OF THE INVENTION The present invention relates to a pistonassembly composed of a skirt defining portion which has opposed thrustfaces and a separate head defining portion including a crown, a ringbelt and grooves therein for receiving piston rings, the skirt definingportion and the head defining portion being disposed in interfittingrelationship to define a wrist pin bore extending through both the skirtdefining and the head defining portions. A bridge or strut member hasits ends anchored in the opposed thrust faces and has a medial portionin confronting relation to the under side of the crown, the bridgemember having a coefficient of thermal expansion less than that of theskirt defining portion. The marginal edges of the medial portion of thebridge member are spaced from the interior of the crown portion, and oneor more apertures are provided in the bridge member to permit oil tocome into contact with view being taken looking through the wrist pinbore; and

FIG. 2 is a cross sectional view taken in a plane 90 from the view ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS defining portion 10 and theskirt defining portion ll are preferably composed of a lightweight, heattransmitting metal such as an aluminum alloy. The head defining portion10 includes a recessed crown portion 12 having a conical portionl3'centrally thereof as is common with some types of heavy duty pistons.The head defining portion 10 includes a ring belt area 14 into whichpiston ring receiving grooves 15 and 16 are provided in the usualmanner. The uppermost piston ring may be received in a groove 17provided in a ring groove reinforcement 18 composed of cast iron or thelike. As explained in Clary US. Pat. No. 3,430,969 such piston ringgroove reinforcements may be received within a circumferential groove 19in tight mechanical engagement, with the junction between thereinforcement 18 and the groove being substantially devoid ofmetallurgical bonding. The reinforcement ring 18 may also includenon-planar opposed faces including raised portions thereon and having anarea to weight ratio of at least square inches of ring in contact withthe piston per pound of ring, thereby reducing the temperaturedifferential between the reinforcement l8 and the piston in service.

' The crown portion of the piston head has an upwardly bowed undersideindicated at reference numeral 20 in the drawings. It also has adepending portion 21 formed.

The skirt forming portion 11 includes a skirt 23 and a wrist pin bore 24which is coaxial with the wrist bore 22 of the head forming portion whenthe head forming portion and the wrist forming portion are placed ininterfitting engagement as shown in FIGS. 1 and 2. Grooves 25 and 26 areprovided in the wrist pin bore 24 for receiving the locking rings (notshown) of the wrist pin assembly itself.

A circumferential groove 27 is provided between the head forming portion10 and the skirt forming portion 11, together with a space 29 toaccommodate relative movement between the two parts. A port 30 isprovided to provide escape for oil wiped off the cylinder wall by thepiston rings, the oil finding its way to the engine oil sump through theclearance provided between the two parts of the piston.

the underside of the crown portion during operation of the piston, andto flow back around the bridge member during further movement of thepiston.

BRIEF DESCRIPTION OF THE DRAWINGS In accordance with the presentinvention, there is provided a bridge or strut member 31 having anupwardly bowed medial portion 32 along which are formed a pair ofupwardly extending flange portions 33 and 34 as best seen in FIG. 2. Thebridge member 31 is composed of steel or other material having acoefficient of thermal expansion lower than the aluminum alloy of whichthe head and skirt forming portions are formed. The extreme ends of thebridge member 31 may be provided with fingers or tabs 35 to facilitateanchoring the ends of the bridge member 31 in the metal through which apartial wrist pin bore 22 is of the skirt forming portion 11, in thevicinity of the thrust faces'36 and 37. Preferably, the fingers 35 areheld in position by having the front of the skirt cast around them. Thebridge member 31 is also provided with one or more apertures 38 so thatupon each downstroke of the piston, oil enters a cooling chamber 39provided between the top of the bridge member 31 and the underside ofthe crown portion of the piston. On each downstroke of the piston, oilenters the chamber 39 and is thrown against the underside20 of thepiston head. On each upstroke of the piston, the oil, which has absorbedheat from the piston crown escapes through the spaces 40 existingbetween the upturned medial flange portions 33 and 34 and theconfronting side wall of the piston head.

The bridge member 31 with its lower rate of thermal expansion than thealuminum piston body restricts the amount of thermal expansion of thealuminum piston skirt. The reduced skirt clearance thus provided reducesthe noise generated when the top of the skirt contacts alternate sidesof the cylinder bore when the side thrust loads reverse direction. Thereduced skirt clearance also reduces the .rate of cavitation erosionwhich occurs on the'outer diameter of wet cylinder liners of the typeused in heavy duty engines.

The bridge member 31 also provides additional strength at the top of theskirt to enableit to carry the side thrust loads encountered in mountedengines without excessive deflection orfailure.

It will be seen that with the piston of the present invention, the headforming portion and the skirt forming portion are retained in theirproper working positions with respect to each other by the wrist pin andthe cylinder bore, but each element is free to rotate about the wristpin to the extent permitted by the clearance between the cylinder boreand the outside diameters of each of the elements. This significantlyreduces the axial misalignment of the ring belt section that isencountered with a conventional one-piece piston and permits the pistonrings to maintain a more effective seal and thereby reduce blow-by andoil consumption. The presence of the bridge member further provides thestrength required to resist the side thrust loads and provides thermalexpansion control at the top of the skirt while also providingadditional strength.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.-

We'claim as our invention:

l. A two-piece metal piston assembly comprising a skirt defining portionhaving opposed thrust faces thereon, a separate head definingportion-having a crown and depending wrist pin bosses and having groovesfor receiving piston rings, said skirt defining portion and said headdefining portion being aligned to provide a wrist pin bore extendingthrough said portions, and a metal bridge member having its endsanchored in said opposed thrust faces, said bridge member having acoefficient of thermal expansion less than that of said skirt definingportion, said bridge member having a large surface area and havingoil-retaining upturned marginal edges facing said crown and being inclosely spaced relation to the crown and to the inner, opposing faces ofthe wrist pin bosses, the space between the bridge member and theunderside of the crown defining a cooling chamber, said bridge memberhaving an aperture therein permitting entry of oil into said coolingchamber upon the downward stroke of said piston. V

2. The two-piece metal piston assembly of claim 1 in which said skirtdefining portion is composed of an aluminum alloy and said bridge memberis composed of steel.

3. The two-piece metal piston assembly of claim 1 in which the ends ofsaid bridge member have finger portions embedded in the metal of saidthrust faces.

1. A two-piece metal piston assembly comprising a skirt defining portionhaving opposed thrust faces thereon, a separate head defining portionhaving a crown and depending wrist pin bosses and having grooves forreceiving piston rings, said skirt defining portion and said headdefining portion being aligned to provide a wrist pin bore extendingthrough said portions, and a metal bridge member having its endsanchored in said opposed thrust faces, said bridge member having acoefficient of thermal expansion less than that of said skirt definingportion, said bridge member having a large surface area and havingoilretaining upturned marginal edges facing said crown and being inclosely spaced relation to the crown and to the inner, opposing faces ofthe wrist pin bosses, the space between the bridge member and theunderside of the crown defining a cooling chamber, said bridge memberhaving an aperture therein permitting entry of oil into said coolingchamber upon the downward stroke of said piston.
 2. The two-piece metalpiston assembly of claim 1 in which said skirt defining portion iscomposed of an aluminum alloy and said bridge member is composed ofsteel.
 3. The two-piece metal piston assembly of claim 1 in which theends of said bridge member have finger portions embedded in the metal ofsaid thrust faces.